Digital signal processing is increasingly utilized in communication systems. In order to digitize the communication signal which is comprised of consecutive symbols or phasors, an interface is required. The traditional interface consists of a balanced, analog, demodulator providing two phase shifted signals followed by two falters and two analog to digital converters (ADCs) to determine both the inphase and quadrature values of each symbol.
For differential phase modulated intermediate (IF) signals, an IF differential detector can be utilized, comprising a .pi./2 phase shifter, a one symbol delay line, two mixers, a low pass filter, a symbol rate sampler, and two ADCs.
An IF differential detector operates as follows: The IF signal is multiplied with a version of itself delayed, via the symbol delay line, by one symbol. The result is passed through the low pass filter and the symbol rate sampler producing the inphase component of the modulated signal. The delayed signal is also passed through the .pi./2 phase shifter and the result is multiplied with the input IF signal. The result is passed through the low pass filter and the symbol rate sampler producing thereby the quadrature component of the demodulated signal.
The inphase and quadrature signals are then converted to digital signals by the ADCs. The phase difference ("phase shift") between two consecutive symbols is computed from the inphase-quadrature pair. All of the components of the IF differential detector are analog except the digital portion of the ADCs.
Other interfaces include a limiter discriminator receiver which comprises a limiter, a discriminator, an integrate and dump filter and an ADC. The IF signal is passed through the limiter and the discriminator and the result is integrated over one symbol period. The resultant signal corresponds to phase differences between consecutive symbols and is sampled once per symbol by the ADC.
The prior art demodulators mix analog and digital elements with ADCs and usually consume a lot of power and are generally difficult to implement and integrate into a digital system.